1. Field of the Invention p The present invention relates to a monolithic, low-noise, synchronous direct detection receiver for passive microwave and millimeter wave radiometric imaging systems, and more particularly to a balanced switched low-noise amplifier (BSLNA) with a front end low-noise amplifier (LNA) to achieve a low-noise figure and provides sufficient gain to minimize the input noise figure degradation due to losses in the switched amplifier circuit, which can be monolithically integrated with a high-electron mobility transistor (HEMT) diode to form a direct detection receiver suitable for use in focal plane array imaging systems.
2. Description of the Prior Art
Radiometric imaging systems are generally known in the art. An example of such a system is disclosed in U.S. Pat. No. 5,438,336, assigned to the same assignee as the assignee of the present invention and hereby incorporated by reference. Such radiometric imaging systems are passive imaging systems, used to sense microwave and millimeter wave radiation, for example thermal radiation, in a field of view known as a focal plane imaging array. The sensed radiation is converted to an output signal to form an image of the object emitting the radiation. Since microwave and millimeter wave radiation is only attenuated to a limited degree by environmental obstacles, such as a fog, haze, light rain, dust and smoke, such imaging systems are useful in various commercial and military applications. For example, as disclosed in U.S. Pat. No. 5,202,692, such radiometric imaging systems are known to be used, for example for aircraft navigation. Such imaging systems are also known to be used to assist aircraft in landing during a low-visibility condition and also would be useful in assisting harbor and channel navigation for ships in fog and various other low-visibility conditions. Such systems are also used to provide images of battle targets during low-visibility conditions, as well as various other applications as generally disclosed in U.S. Pat. No. 5,198,776.
Such radiometric imaging systems are known to utilize different types of receivers to detect the microwave and millimeter radiation. For example, both direct detection receivers, as well as heterodyne-type receivers, are known. For example, FIGS. 4 and 5 of U.S. Pat. No. 5,202,692 illustrate known heterodyne-type receivers used with radiometric imaging systems. Unfortunately, such heterodyne receivers require a local oscillator, which increases the number of hardware components of the receiver and also adds to the total power requirement. In addition, local oscillators are known to be relatively bulky, which make them generally unsuitable for monolithic integration.
As such, direct detection receivers are known to be used that obviate the need for a local oscillator. An example of such a direct detection receiver is disclosed in U.S. Pat. No. 4,557,272. That system utilizes a so-called Dicke switch for comparing the radiation from the unknown source with a reference signal. The Dicke switch serves to switch the receiver input from the antennae receiving the electromagnetic radiation from the unknown source and the known source at a constant rate. The switched or modulated signal is then amplified and detected. Dicke switches are discussed in detail in "The Measurement of Thermal Radiation at Microwave Frequencies," by R. H. Dicke, The Review of Scientific Instruments, Vol. 17, No. 7, July 1946. Direct detection receivers that utilize Dicke switches are not suitable for monolithic integration. Other drawbacks of direct-detection receivers utilizing such Dicke switches are generally discussed in U.S. Pat. No. 5,149,198.
Direct detection receivers for radiometric imaging systems that utilize so-called Dicke switch replacements are also known. An example of such a system is disclosed in U.S. Pat. No. 5,149,198. The so-called Dicke switch replacement disclosed in the '198 patent includes a pair of hybrid rings defining a pair of input ports and a pair of output ports. The input ports are adapted to be coupled to the source of radiation to be detected, as well as to a reference source. Two parallel paths are formed between the input port and the output port. In one path, a pair of amplifiers is coupled to a 0-180.degree. phase-shift switch, which, in turn, is connected to an output port. In the alternative path, two amplifiers are connected between the input port and the output port. One advantage of the Dicke switch replacement illustrated in the '198 patent is that the signal is amplified prior to being switched, thus improving the signal-to-noise ratio of the radiometer. However, with such a configuration, the loss contribution of the hybrid ring is relatively significant, which tends to degrade the receiver temperature sensitivity.